(a) Field of the Invention
The present invention relates to a method for controlling an AC motor. More particularly, the present invention relates to a method for controlling an AC motor that is able to drive the AC motor stably and efficiently operating at voltage limit such as a high-speed operation and inverter dc-link voltage shortages.
(b) Description of the Related Art
Recently, according to the development of driving technology for a variable speed motor, the application of variable speed motors is increasing rapidly such as ultra-high-speed (>120,000 r/min), high efficiency, and high-power-density drives, such as electrically-assisted turbo chargers, turbo compressors, white goods applications, and reduced dc-link capacitor technology for heating-ventilating-air-conditioning (HVAC) systems. A driving system for the variable speed motor has high efficiency and low vibration. In addition, when the variable speed motor is driven at high-speed, plant productivity may be improved, and the variable speed motor may be driven in direct connection way without a separate increase speed gear, thereby reducing mechanical noise. Particularly, since an AC motor including a permanent magnet synchronous motor (PMSM) has excellent characteristics in efficiency, it is widely used in high-speed applications.
However, a speed sensor is required to perform a wide range of speed control for the variable speed AC motor, but the position sensor has structural reliability drawbacks and requires extra cost. In addition, a motor design process and a machining process are complicated due to the position sensor installment. Further, when the position sensor is installed, an axial direction length of the motor increases, such that a reduction in the size of an entire system may be difficult.
Accordingly, since the variable speed AC motor has such drawbacks, a position sensorless operation for the variable speed motor is required in the high-speed applications.
Further, since conventional current control methods therefor should be used only in linear voltage limits at high-speed, efficiency may be deteriorated, and a closed-loop current control may be unstable due to feedback sampling delay and limited sampling frequencies at elevated speeds for the closed-loop current control because the inverter switching frequency cannot be made sufficiently high, compared to the fundamental frequency.
A background technique of the present invention is disclosed in Korean Patent Laid-Open Publication No. 2007-0107539 (2007 Nov. 7).
The above information disclosed in this Background section is only to enhance the understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.